A large number of rifles, shotguns, muzzleloaders, military firearms, etc. were and are manufactured equipped only with open sights, also called iron sights. These rifles may be subsequently fitted with optical sights such as riflescopes, reflex, and holographic sights, but in order to be accurate, these optical sights must “zeroed in.” A bullet fired from a rifle or other firearm that is zeroed in will strike a target at the aimpoint at a specific distance, given a good shooter and good shooting conditions. The process of zeroing a firearm is most commonly referred to as sighting-in. Unfortunately, current methods for sighting-in can be costly, imprecise, and/or require much time and ammunition.
One method for sighting-in involves mounting the optical sight on a rifle, and then centering the elevation and lateral aimpoint adjustment screws on the optical sight and test firing at a paper target (herein, the aimpoint on a target is assumed to be a bullseye at the center of the target). This method does not give the shooter a good idea of the flightpath of the bullet. Consequently, the test bullet may miss the target altogether, sometimes leaving the shooter with no idea where the bullet went.
In response, four aiming tools were developed to assist with the sighting-in process: (1) boresighting; (2) optical collimators; (3) laser devices; and (4) sight-to-sight co-alignment. These four aiming tools all work on the same principle: the optical sight is aimed at a point on a target determined by aiming with the bore or open sights of the firearm or at a point on a target that emanates from a laser or an optical collimator mounted on the firearm. The four aiming tools help ensure that a test bullet will hit somewhere on a paper target, known as getting “on paper,” if not close to the aimpoint. Getting on paper gives a reference point for correcting the aimpoint of the optical sight. Each of these aiming tools will be examined in greater detail later.
Two aimpoint-correction methods can be used to correct the aimpoint of an optical sight: (1) measure-and-adjust; or (2) adjust-to-strikepoint. Both methods assume that an aiming tool was used to initially adjust the optical sight, and that a carefully fired test shot hit the target. Either method can then be used to move the optical sight's aimpoint until it corresponds with the strikepoint of the test bullet. To use the measure-and-adjust method, a shooter must know how turning the elevation and lateral adjustment screws on the optical sight will move the aimpoint of the optical sight at the distance to the target. For example, turning the elevation and lateral screws one click may move the aimpoint 0.25 inches at 25 yards. The shooter then measures the horizontal and vertical distances from the aimpoint to the bullet hole on the target. He/she then adjusts the elevation and lateral adjustment screws the appropriate number of clicks to remove the aiming error. The test firing and aimpoint-correction process may need to be repeated because the actual movement of the aimpoint per click of the elevation and lateral adjustment screws often differs from the manufacturer's specifications, especially with less expensive optical sights or when the adjustment mechanisms are near their limits of adjustment.
The other aimpoint-correction method, adjust-to-strikepoint, requires that the shooter be able to see the strikepoint of the test bullet with the optical sight from the shooting position. If not, an easy-to-see marker, such as a black disk, can be positioned over the strikepoint. Then, the shooter steadies the rifle using a rifle rest and/or sandbags and aims at the target with the aiming tool. The shooter then adjusts the optical sight so that it aims at the strikepoint of the test bullet while maintaining the aim of the aiming tool on the target. This adjust-to-strikepoint method adjusts the optical sight so that it aims at the point where test shots strike the target—which is the goal of sighting-in. The adjust-to-strikepoint method, however, has disadvantages. Unlike the measure-and-adjust method, the adjust-to-strikepoint method must be used with a boresighting, laser device, or sight-to-sight co-alignment aiming tool, each of which have limitations. The adjust-to-strikepoint method, moreover, cannot be used with a collimator aiming tool. The four aiming tools will be discussed in detail later. Overall, the adjust-to-strikepoint aimpoint-correction method is significantly more efficient than the measure-and-adjust method. There is no need to measure the amount of aiming error thereby negating the need for tape measures or graduated targets. If the shooter is on a firing range with other shooters and can see the bullet hole, there is no need to ask the other shooters to cease fire in order to safely walk to the target to make measurements. Furthermore, no mathematical computations are necessary because a shooter need not know the distance that each elevation and/or lateral adjustment moves the aimpoint at that distance, which may not be as the manufacturer states. These advantages remove many potential sources of error; thereby saving time, ammunition, and frustration.
Although the four aiming tools are generally reliable, they suffer from a number of deficiencies that can produce significant aiming errors. Therefore, most shooters first use them to help sight-in at a relatively short distance, for instance, 25 yards. Completing the sighting in process at 25 yards using live fire ensures that test shots fired at a target at a longer distance will be on paper. That longer distance is herein referred to as the “zero distance.” The zero distance is chosen by the shooter for specific use, such as 100 yards to hunt whitetail deer or 200 yards or more to hunt antelope or elk. A rifle sighted-in at the zero distance is said to be zeroed. Gravity begins to pull a bullet downward as soon as it leaves the barrel; therefore, to achieve a zero at, for example, 200 yards, the barrel of a rifle must point up relative to the line-of-sight. The line-of-sight, or center line, of most riflescopes averages about 1.5 inches above the bore of the barrel. The upward cant of the barrel causes a bullet to first rise relative to a riflescope's line of sight, reach an apex, and then fall through the line-sight at exactly the zero distance. Bullets fired from most modern high-powered rifles that are zeroed for a significant distance rise through a riflescope's line of sight before or after 25 yards. A shooter may elect to set the aimpoint of the riflescope above or below the strikepoint of a test bullet on a target positioned at 25 yards so that a test bullet will fall down to the aimpoint on a target positioned at the zero distance, how far above or below can be determined from trajectory tables found in books or on the Internet. This vertical aimpoint offset is herein referred to as “target to zero-distance aimpoint compensation” or TZDAC. Using TZDAC at 25 yards helps ensure that test bullets fired at a target positioned at the zero distance will strike close to the aimpoint. However, doing so should never be relied on to yield satisfactory results. Trajectory tables are commonly based on both live fire tests made by factory marksmen and computed trajectories, which yield good but not perfect results. More importantly, the factory marksmen may have used a rifle that differs significantly from that used by the current shooter, including length and condition of the barrel. Differences in altitude and performance of ammunition made by different manufacturers can also cause the trajectory of tests bullets to differ from that shown on trajectory tables. In addition, any aimpoint errors made at 25 yards will be magnified at the longer distances. For these reasons, the most prudent shooters fire test shots at the zero distance and make the necessary aimpoint corrections. Other shooters use live fire results and TZDAC at 100 yards as an acceptable means of ensuring a satisfactory zero at a longer distance, say 200 yards. They set the strikepoint of test bullets 2-3 inches higher than the aimpoint at 100 yards knowing from the trajectory tables that the bullets will drop 2-3 inches (down to the line-of-sight) over the next 100 yards. One bullet that rises very close to 1.5 inches (i.e., to the line of sight of the riflescope) at 25 yards, and which falls through the line-of-sight at 200 yards is the popular 0.300 Savage, 150-grain pointed soft point Core-Lokt made by Remington Arms Company. In this document, for reasons of simplicity, it is assumed that the rifle being sighted in fires that cartridge, and that the first test shots are at a target 25 yards away and that the zero distance is 200 yards. Therefore, any aimpoint-corrections at those distances can be made to the strikepoint of the test bullets and use of TZDAC is unnecessary.
BORESIGHTING Use of the four aiming tools for sighting-in, including checking that the optical sights have maintained their zero, is now described. The first aiming tool, boresighting, works with rifles and other firearms that allow a shooter to aim with the bore by looking through it from the breech end. Firearm types that allow this are those with break open, falling block, bolt actions, and muzzleloaders with removable breech plugs. Firearm types that preclude looking through the bore from the breech end include those with semi-automatic, pump, and lever actions. In those cases, a device that incorporates a right-angle prism or small mirror set at a 45-degree angle can be fitted into the action to provide a view down the bore from the top or side of the action. The bore is aimed at a target, usually 25 yards away. Then, the rifle is steadied and the optical sight is adjusted until it also aims at the target, all the while ensuring that the bore remains aimed at the target. The initial static stage of the sighting-in process is now complete and the live-fire stages begin. Using a rifle rest and/or sandbags to ensure accuracy, a test shot is fired at the target using the optical sight. The probability of getting on paper at this short distance is very high; however, it is also probable the test bullet will miss the aimpoint by some measure. Thus, a shooter can then use either the measure-and-adjust method or the adjust-to-strikepoint method to remove the aiming error. To use the preferred adjust-to-strikepoint method, the bore is aimed at the target and the rifle is steadied. Then the optical sight is adjusted to aim at the strikepoint of the test bullet, while ensuring that the bore remains aimed at the target. Then another test shot is fired to ensure that the bullet hits the aimpoint or very close to it.
The sighting-in process is usually completed by carefully firing three shots at a target at the zero distance and marking the centerpoint of the shot-group. If the zero distance is more than one hundred yards and the centerpoint of the shot-group is relatively close to the aimpoint, a shooter can use the measure-and-adjust method to remove the aiming error. Boresighting is too imprecise to use the adjust-to-strikepoint method for final zero, especially if the firearm has a large diameter bore such as muzzle loaders and shotguns. If all goes well, the sighting-in process is now complete.
To later check that the optical sight has retained its zero, a shooter notes the position of the aimpoint of the bore relative to the aimpoint of the optical sight on a target at the zero or other distance. Thereafter, he/she need only check that the aimpoints of the bore and the optical sight retain that spatial relationship at that distance. Because boresighting can be relatively imprecise, especially with large bore firearms, this method can reliably detect only moderate to gross changes in the aimpoint of the optical sight. Boresighting has other disadvantages. As previously noted, right-angle prisms and mirrors must be used with some types of firearms and these devices add cost and require aiming from the side or top, which is unnatural and can be difficult for some shooters.
COLLIMATORS The second aiming tool uses an optical collimator as the key component. A collimator is comprised of a short tube containing a convex lens at one end and a small opening at the other viewing end, which is at the focus length of the lens. Collimators produce a beam of parallel light rays that appear to emanate from a point an infinite distance away. A grid incorporated into a collimator can be seen only when looking straight at the viewing end of a collimator. Most collimators are held upright by an extendable metal rod, the bottom portion of which connects to a stud at a right angle. A shooter snugly inserts the stud into the muzzle and then rotates the collimator assembly around the axis of the stud and extends the rod, if necessary, until the collimator stands squarely in front of the optical sight. Once this is completed, the grid of the collimator will appear centered in the ocular lens of the optical sight. Instead of muzzle studs and extendable rods, some manufacturers mount the collimator on the top of a flat-faced plastic arm that has two thin magnets embedded along its length. The magnetic arm is positioned vertically on the muzzle of the rifle and then slid up or down on the muzzle until the collimator stands squarely before the optical sight. After a collimator is correctly positioned, the optical sight is adjusted so that it aims at the center point of the grid. The collimator is then removed, and the optical sight is used to fire one or more test shots at a target 25 yards away. The aimpoint is then corrected using the measure-and-adjust method. The more efficient adjust-to-strikepoint method cannot be used because it requires aiming at the target with the aiming tool, which is impossible with a collimator.
To finish sighting-in, three or more test shots are fired at the target at the zero distance and the centerpoint of the shot group is marked. The measure-and-adjust method can then be used again to make any necessary corrections to the aimpoint of the optical sight at that distance.
After the optical sight is zeroed, a shooter can take certain steps that will enable him/her to later check that the optical sight has retained its zero. To do so, a shooter positions the collimator on the muzzle again and notes the optical sight's aimpoint on the grid on the collimator. Thereafter, he/she can mount the collimator on the barrel to check if the optical sight still aims at the same point on the grid. Unfortunately, the aimpoint seldom falls on a vertical or horizontal line gridline, much less an intersection of such and, therefore, one must estimate when trying to determine the location of the original aimpoint on the grid and when trying to return to it, which leads to error. Collimator aiming tools have a number of other disadvantages. Sizing is especially a problem. Manufacturers of collimators that use studs that fit into the muzzle must supply a significant number of different sizes to match different caliber barrels. To get around this problem, some manufacturers use expandable arbors or other sizing devices. Proper positioning of the collimator can also be a problem. Collimators should always be positioned vertically. To do so, most manufacturers recommend that the collimator be adjusted until the horizontal and vertical lines on the grid are parallel to the horizontal and vertical wires on the reticle of the optical sight; however, this is impossible with optical sights that use dot rather than crosshair reticles. The magnets used on some collimators can also cause problems. Some hunters carry collimators with magnetic arms in the field and these should be kept well away from compasses. Finally, the collimators must be significantly precise, which makes them relatively expensive.
LASERS A third aiming tool uses lasers devices, of which a number of sub-types exist. One sub-type fits all the electronic components into a cartridge that fits into the chamber of the firearm. Closing the action of the firearm turns on the laser, which then projects its beam through the bore. A second sub-type consists of a casing that is inserted into the chamber of the firearm; however, the power supply and other electronics are contained in an external housing connected to the casing by wires. Some manufacturers of this latter type laser aiming tool include a short cylinder with sizing devices that is inserted into the muzzle. A hole that runs lengthwise through the cylinder helps align the laser beam by allowing only that part of the laser beam that is centered to pass while blocking the rest. A third sub-type laser aiming tool fits all the components into housings shaped like disks or cylinders with a stud projecting from the back. The stud is aligned back-to-back with the laser. A shooter inserts the stud into the muzzle of the firearm and turns on the laser which projects its beam in a direction directly opposite that of the stud or muzzle. Some manufacturers include a magnet within the stud or around the base of the stud where it extends from the housing to pull the stud or housing against the crown of the muzzle. This prevents the housing from canting downward due to its weight. A fourth sub-type uses only a super strong magnet to hold the housing to the muzzle; no stud is required.
To use any of the laser aiming tools, the laser cartridge or casings are inserted into the chamber of the firearm, or the devices are mounted in or on the muzzle. The firearm is moved until the laser spot is on a target at a distance at which the spot can easily be seen, usually 25 yards or less in direct sunlight. The optical sight is then adjusted until it aims at the laser spot. There is no need to steady the rifle because the laser spot can be seen through the optical sight. The laser device is then removed, a test shot is fired at the target, and the adjust-to-strikepoint method is then preferably used to remove the aiming error. To do so, the laser device is mounted on the rifle again, and the laser spot is held on the target while the optical sight is adjusted to aim at the strikepoint of the test bullet. The laser device is removed again, a test shot is fired, and the necessary aimpoint corrections are made. To finish sighting-in, three or more test shots are fired at a target positioned at the zero distance and the center point of the shot group is marked. In normal daylight, a shooter probably will not be able to see the laser spot on the target, so he/she must use the measure-and-adjust method to remove the aiming error. Assuming all goes well, the sighting-in process will be complete.
To later check that the firearm's optical sight has maintained its zero, the laser device is mounted in/on the rifle again and aimed at a target positioned at a distance at which the laser spot can easily be seen under almost all lighting conditions, usually no more than 25 yards. The position of the laser spot relative to the aimpoint of the optical sight is then noted. Thereafter, a shooter can check that the optical sight has maintained its zero by checking that its aimpoint retains its position relative to the laser spot on a target positioned at that distance.
Laser aiming tools have a number of disadvantages. No cartridge lasers exist for the popular 22-caliber and 17-caliber magnum cartridges because the electrical components cannot be fitted into such small containers. In addition, manufacturers must provide matching laser cartridges for bullets of different shapes and sizes. Some manufacturers of laser cartridges fit all the electronics components into a single cartridge of the smallest size they provide, but the cartridge can be fitted with sleeves to give it size and shape of other larger bullets. Muzzle-aligned laser boresighters also have sizing problems. To ensure a good fit for different size bores, some manufacturers provide matching studs while others provide expandable arbors, magnets within arbors, tapered self-centering bushings, and other devices. All of the sizing solutions add cost and are inconvenient. The magnets used with some of the devices can also cause problems. Magnets used with housings, whether alone or with a stud, can cause the housing to tilt and, therefore, send the laser beam off line. This can occur if a stud does not fit snugly in the muzzle or if the muzzle crown has been damaged or is otherwise not at perfect right angles to the axis of the bore. Safety is of special concern with laser devices that fit into the muzzle. These devices must be removed before firing a test shot since not doing so can damage the firearm and even cause severe injury or death. Muzzle-mounted disk-shaped lasers and optical collimators give shooters a second chance to remove the devices because it is obvious to shooters that the devices are still attached when they aim through the optical sight to take a test shot. Unfortunately, this is not the case with some of the small cylindrical laser aiming tools that fit into the muzzle, especially when they are concealed by hooded front sights. In addition, the electronic components; sizing factors, and the necessary precision in manufacturing all contribute to the relatively high initial cost of laser aiming tools. There is also the cost and inconvenience of replacing batteries. A final disadvantage of laser aiming tools is the limit on usable distances caused by inability to see the laser spot in bright light. A laser spot can be seen up to 500 feet indoors or outside in early morning or late evening on dark cloudy days, but only to about 75 feet outside in normal daylight and even less in bright sunlight. Shades can be placed over targets on bright days to make the spot more visible, but this adds hassle. Some manufacturers provide reflective targets and/or laser enhancement glasses to increase the visibility of the laser spot, but this adds cost. Riflescopes help a shooter to see the laser spot relative to their power of magnification, but holographic and most reflex sights do not magnify the image.
Finally, the three aiming tools described above, i.e., boresighting, collimators, and laser devices, suffer from one additional disadvantage during their initial static use: all are based on the emanation of a straight line. As such, none of the three take into direct account the actual flightpath of a bullet fired from the firearm being sighted in. Gravity, air resistance, powder type and weight, bullet shape and weight, barrel length, and other factors affect a bullet such that its flightpath is not straight, but parabolic—the exact form of which is determined by the interplay of the above factors. In addition, bullet type and weight, powder type and weight, rate of twist, bore tolerance, barrel stiffness, fit of the barrel and other metal parts onto the stock, and other factors cause various degrees of so-called “muzzle whip.” Muzzle whip and recoil occur together. The effects of both combine to “throw” a bullet off line mostly in a vertical direction; however, there is usually some lateral throw as well. All of the above disadvantages, mutual or specific, explain why most prudent shooters consider the three aiming tools generally suitable only for getting “on paper” first at a relatively short distance, e.g., 25 yards, and then making the necessary aiming corrections before progressing to a target farther away.
SIGHT-TO-SIGHT CO-ALIGNMENT The fourth aiming tool is sight-to-sight co-alignment, of which a number of sub-types exist. One sub-type requires that a riflescope be mounted on see-through mounts (or the much less common side mounts), and that the rifle be equipped with open sights. See-through riflescope mounts enable one to either use the riflescope or the open sights, the latter by looking under the riflescope through a window in the scope mounts.
Using see-through mounts to co-align the aimpoint of a riflescope with that of open sights that are known to be well-zeroed is simple. First, the open sights are aimed at a target at the zero distance. Then, the rifle is steadied and the riflescope is adjusted to aim at the same point on the target as the open sights. Any test shots fired thereafter using the riflescope can be expected to hit close to the aimpoint of the open sights at the zero distance. However, to finish sighting-in, a shooter should still fire three or more test shots at the target using the riflescope. A shooter can aim better using a riflescope than with open sights, especially if he/she is older or has impaired vision. Therefore, the centerpoint of a three-shot group acquired with the riflescope may differ somewhat from that acquired with the open sights, and some tweaking of the riflescope's aimpoint may be in order.
Sight-to-sight co-alignment has a number of other advantages over the other aiming tools. The method is highly specific to the rifle being sighted-in because it co-aligns the aimpoint of the riflescope to the aimpoint of the open sights that were sighted-in by live-fire at the zero distance. This fact negates the need to sight-in at a short distance first, thereby saving time and ammunition. The sight-to-sight co-alignment method is accurate and the results are highly repeatable. Furthermore, the efficient adjust-to-strikepoint aimpoint-correction method works well with sight-to-sight co-alignment. Checking that the riflescope and the open sights remain co-aligned as a means of determining whether one or the other is out of adjustment is fast and simple. The rifle is steadied and the riflescope and the opens sights are checked to determine if they remain aimed at the same point at the zero distance. Actions do not have to be opened, bolts do not have to be removed, collimators or laser boresighters do not have to be positioned on the rifle, and notes on the spatial relationship of the aimpoints of the riflescope and other aiming tools on a target need not be consulted. A shooter need only remember the zero distance to check that the elevation setting is correct. If the riflescope is still zeroed, then the lateral setting will be correct at any distance. Sight-to-sight co-alignment using see-through mounts is also relatively inexpensive; requiring only the purchase of the mounts, but most shooters buy see-though mounts so that they can use the open sights as backup, and so that cost is spread out.
Unfortunately, see-through mounts have a number of disadvantages. In order to allow a shooter to see under the riflescope, see-through mounts must elevate riflescopes significantly more than regular mounts. The disadvantages of the added height include: (a) riflescopes are more likely to be damaged or knocked out of adjustment because their added height increases the chances that they will be knocked about and also because the added height of the riflescopes causes additional forces to be applied to their mounts, bases, and screws; (b) the height of the riflescope affects the balance of a rifle, making it feel “tippy”, and, hence, more tiresome to carry and awkward to use; (c), the height of riflescopes require shooters to hold their heads higher than normal when sighting. This heads-up position can prevent shooters from getting a good “cheek-weld” on the comb of the stock which can cause delay and/or errors in aiming because of an unstable head position and/or parallax error. A shooter can always add a laced-on leather cheek rest to the comb of the stock, but this adds cost and weight and detracts from the aesthetics and the balance of the rifle; and (d) the height of the riflescope makes the rifle difficult or impossible to fit into some gun cases and scabbards. Finally, as stated previously, another and probably the most common reason shooters use see-through mounts is so they can use the open sights if their riflescopes become unusable. This presumption, however, does not always work as intended. For example, in snapshooting at running deer or elk, many hunters find that their shooting eye will go to the riflescope by force of habit, even when they consciously know it is unusable. And by the time they realize they can use the open sights, the opportunity for a shot is often gone. In addition, see-through mounts restrict the field-of-view around the open sights, which makes snapshooting all the more difficult. Instead, experienced hunters simply remove their optical sights if they become unusable so they can then use their open sights. For all the above reasons, most shooters disdain the use of see-through mounts and use regular low mounts instead.
Besides see-through mounts, two other sight-to-sight co-alignment systems exist. One system uses open sights mounted on the top of a riflescope. The rear sight is usually an aperture sight mounted on the top of the ocular bell and the front sight is a blade mounted on the top of the objective bell. These open sights have two primary disadvantages. First, most are non-adjustable, and, therefore, they rely on the mechanical mounting of the riflescope to ensure accuracy, an objective that meets with mixed results. Second, they are plagued by sighting errors because of their short sighting radius. Hence, most of these open sight systems are designed only as back-up sights suitable only for emergency use, such as in combat. One available example is the ACOG 4X32 riflescope, model TA01NSN, manufactured by TRIJICON. These systems can be used for sight-to-sight co-alignment, but only at relatively short distances.
The remaining sight-to-sight co-alignment system uses open sights in conjunction with non-magnifying optical sights, like the HOLOSIGHT made by BUSHNELL or the RD30 RED-DOT made by BSA. These optical sights incorporate a small, battery-powered laser that imposes a reticle, usually a red-dot, on a transparent glass plate within the sight. The open sights usually consist of a front blade sight that is mounted on the barrel in front of the optical sight and a rear aperture sight that is mounted behind the optical. Because the optical sight is non-magnifying, a shooter can use the open sights. The effect is like having two thin panes of transparent glass positioned between the rear aperture sight and the front blade sight. Normally, the shooter will use the optical sight, but when it is damaged or when the batteries lose power, the shooter can switch to the open sights. This dual-sight or “co-witness” system can be used for sight-to-sight co-alignment. However, two main disadvantages exist: (1) the system can only be used with non-magnifying optical sights, and (2) the view through the majority of these optical sights is above the line-of-sight of most open sights of normal height. However, the elevated sights on some military rifles like the M-16, the main U.S. military rifle, enable sight-to-sight co-alignment. There are two methods of mounting a non-magnifying optical sight on the M-16 rifle so that the open sights can also be used. One method uses a special mount that positions the optical sight in front of and somewhat below the rifle's carrying handle in line with and between the rear aperture sight (which is mounted on the rifle's carrying handle) and the elevated front sight. The other method is to mount the optical sight on a variation of the M-16 rifle that does not have a carrying handle. In this case, the optical sight is mounted over the receiver at a height which places it in line with and between the rear aperture sight and the elevated front sight. The rear aperture sights partially obstruct a shooter's view through the primary optical sight, but this problem can be solved by using a type of aperture sight that can be folded down, but these are costly. An elevated rear notch sight mounted in front of the optical sight could also be used in place of the rear aperture sight, but none are known to be available.
The sight-to-sight co-alignment method using see-through mounts or non-magnifying optical sights can also be used to zero open sights. For example, a shooter may adjust the aimpoint of a newly mounted optical sight to the aimpoint of the open sights, which he/she thinks are well zeroed—but are not. A test shot using the optical sight, may, for example, hit 5 inches from the aimpoint of a target positioned at 25 yards, which translates to an aimpoint error of 20 inches at 100 yards. This means that the open sights were not zeroed. Nevertheless, as long as the strikepoint of the test bullet is on paper, the adjust-to-strikepoint method can be preferably used to zero the optical sight. After the optical sight is zeroed, the shooter can then elect to use the sight-to-sight co-alignment method to correct the aimpoint of the open sights. To do so, the shooter steadies the rifle and then adjusts the aimpoint of the open sights to the aimpoint of the optical sight at the zero distance. This is, of course, the reverse of the usual use of the sight-to-sight co-alignment method. Zeroing open sights by this method offers definite advantages. Zeroing open sights using other methods can be difficult. Usually, only the rear open sights are adjustable, and most are crude devices that are difficult to adjust. Most have dovetail bases that fit into cutouts on the barrel. Lateral adjustments are usually made by placing a brass punch on the side of the dovetail base and tapping it with a hammer. This drives the rear sight right or left by force of impact. Elevation adjustments are usually made by loosening one or more set screws on the rear sight and moving it up or down on a ramp or in a slot. Often, these elevation and lateral adjustments yield too much or too little movement of the aimpoint, and adjustment markings on the sight, if any, are of little help. These deficiencies make the zeroing of open sights a trial-and-error process because a shooter cannot easily tell if he/she over corrected or under corrected or by what amount. This means that numerous test shots may be required before the open sights are zeroed. Using the aimpoint of an already zeroed riflescope as a reference, a shooter can easily tell if the open sights were moved in the correct direction and by the proper amount and, if not, to try again. Therefore, zeroing open sights by co-aligning their aimpoint to that of the aimpoint of a riflescope that is already zeroed can save much time, ammunition, and frustration.
One disadvantage common to all sight-to-sight co-alignment methods is that the firearm must be equipped with open sights. Some current firearm manufacturers of rifles include open sights, but some do not in order to reduce costs and because they expect the rifle to be equipped with optical sights, but this practice deprives a shooter of backup sights when the optical sights are rendered unusable. The practice by some current firearm manufacturers of not including open sights does not apply to rifled shotguns, muzzleloaders, and military firearms manufactured throughout the world. All in all, many tens of millions of such firearms are equipped with open sights.
There is thus a need for an apparatus and a method to sight-in that is safe, accurate, highly repeatable, useable under most lighting conditions, is efficient in its use of ammunition, and does not require batteries.